Some conventional hearing protection systems that target airborne vibrations include passive hearing protection systems and active hearing protection systems.
Conventional passive hearing protection systems rely on blocking airborne sound waves from entering the middle and inner ear of a user. Examples include earmuffs, earplugs, and the like. Conventional passive hearing systems typically offer about 22-24 dB of protection.
Conventional active-noise reduction (ANR) hearing systems typically rely on generating sound waves with the same amplitude and opposite polarity (180° out of phase) to the original sound waves. The original sound waves are typically recorded with microphones, electronically processed and cancellation sound waves are output from a transducer or speaker. The cancellation waves may be centered on a certain frequencies and may be tailored for different applications, e.g., an airplane cabin noise, engine propeller noise, resident frequencies, and the like. Central to ANR is the speaker effectiveness resulting from its placement and orientation. ANR typically offers an additional 20-22 dB of protection to passive hearing protection system.
The conventional hearing protection systems discussed above protect only against airborne sound vibrations. However, bone conducted sound vibrations can cause significant damage to the middle and inner ear. Three mechanisms by which bone conducted vibrations coupled to the inner ear and translated into sound include: 1) the vibrations can squeeze the ear canal creating vibrations in the ear within the canal (this serves to reinforce airborne vibrations and amplify ambient sound), 2) bone vibrations can cause the ear drum and/or inner ear bones to vibrate, mimicking the effect of air coupled vibrations, and 3) the vibrations of the structures surrounding the inner ear can cause hair cells themselves to vibrate, causing them to fire and create a direct perception of sound. All of the above are fairly local phenomena, i.e. no matter at which site the initial sound is coupled into the anatomy, it is when the induced bone conducted vibrations travel to the middle and inner ear that it becomes “sound”, and perhaps more importantly, where the bone conducted vibrations can damage the fine structures of the middle and inner ear.
Conventional hearing protection systems which attempt to solve the problem of hearing loss due to bone conducted vibrations have resulted in varying levels of success. Other conventional hearing protection systems may rely on passive bone conducted sound attenuation. However, such conventional hearing protection systems attenuate vibrations coupled into the entire skull and are cumbersome and uncomfortable to wear.